EP3337923B2 - Fiber reinforced tissue composites - Google Patents

Description

BACKGROUND
• Leather is used in a vast variety of applications, including furniture upholstery, clothing, shoes, luggage, handbag and accessories, and automotive applications. Currently, skins of animals are used as raw materials for natural leather. However, skins from livestock pose environmental concerns because raising livestock requires enormous amounts of feed, pastureland, water, and fossil fuel. Livestock also produces significant pollution for the air and waterways. In addition, use of animal skins to produce leather is objectionable to socially conscious individuals. The global leather industry slaughters more than a billion animals per year. Most of the leather comes from countries with no animal welfare laws or have laws that go largely or completely unenforced. Leather produced without killing animals would have tremendous fashion novelty and appeal.
• Although synthetic leather was developed to address some of these concerns, it lacks the quality, durability, and prestige of natural leather. Thus far, scientifically sound and industrially feasible processes have not been developed to produce natural leather. Accordingly, there is a need for a solution to demands for alternative to leather produced from live animals.
• Natural leather is typically a durable and flexible material created by the tanning of animal rawhide and skin, often cattle hide. Tanning is generally understood to be the process of treating the skins of animals to produce leather. Tanning may be performed in any number of well-understood ways, including vegetable tanning (e.g., using tannin), chrome tanning (chromium salts including chromium sulfate), aldehyde tanning (using glutaraldehyde or oxazolidine compounds), syntans (synthetic tannins, using aromatic polymers), and the like.
• Natural leather is typically prepared in three main parts: preparatory stages, tanning, and crusting. Surface coating may also be included. The preparatory stages prepare the hide/skin for tanning, and unwanted raw skin components are removed. The preparatory stages may include: preservation, soaking (rehydrating), liming, de-hairing, de-fleshing (removing subcutaneous material), splitting, re-liming, deliming (to remove de-hairing and liming chemicals), bating (protein proteolysis), degreasing, frizzing, bleaching, pickling (changing pH), de-pickling, etc.
• Tanning is performed to convert proteins in the hide/skin into a stable material that will not putrefy, while allowing the material to remain flexible. Chromium is the most commonly used tanning material. The pH of the skin/hide may be adjusted (e.g., lowered, e.g. to pH 2.8-3.2) to enhance the tanning; following tanning the pH may be raised ("basification" to a slightly higher level, e.g., pH 3.8-4.2).
• Crusting refers to the post-tanning treatment that may include coloring (dying), thinning, drying or hydrating, and the like. Examples of crusting techniques include: wetting (rehydrating), sammying (drying), splitting (into thinner layers), shaving, neutralization (adjusting pH to more neutral level), retanning, dyeing, fatliquoring, filling, stuffing, stripping, whitening, fixation of unbound chemicals, setting, conditioning, softening, buffing, etc.
• In practice, the process of converting animal skin into leather may include sequential steps such as: unhairing/dehairing, liming, deliming and bateing, pickling, tanning, neutralizing/Dyeing and Fat liquoring, drying and finishing. The dehairing process may chemically remove the hair (e.g., using an alkali solution), while the liming step (e.g., using an alkali and sulfide solution) may further complete the hair removal process and swell ("open up") the collagen. During tanning, the skin structure may be stabilized in the "open" form by replacing some of the collagen with complex ions of chromium. Depending on the compounds used, the color and texture of the leather may change. Tanned leather may be much more flexible than an untreated hide, and also more durable.
• Skin, or animal hide, is formed primarily of collagen, a fibrous protein. Collagen is a generic term for a family of at least 28 distinct collagen types; animal skin is typicallytype 1 collagen (so the term collagen is typically assumed to betype 1 collagen), although other types of collagen may be used in forming leather. Collagens are characterized by a repeating triplet of amino acids, -(Gly-X-Y)_n-, so that approximately one-third of the amino acid residues are in collagen are glycine. X is often proline and Y is often hydroxyproline. Thus, the structure of collagen may consist of twined triple units of peptide chains of differing lengths. Different animals may produce different amino acid compositions of the collagen, which may result in different properties (and differences in the resulting leather). Collagen fiber monomers may be produced from alpha-chains of about 1050 amino acids long, so that the triple helix takes the form of a rod of about 300 nm long, with a diameter of 1.5 nm. In the production of extracellular matrix by fibroblast skin cells, triple helix monomers may be synthesized and the monomers may self-assemble into a fibrous form. These triple helices may be held together by salt links, hydrogen bonding, hydrophobic bonding, and covalent bonding. Triple helices can be bound together in bundles called fibrils, and fibril bundles come together to create fibers. Fibers typically divide and join with each other throughout a layer of skin. Variations of the crosslinking or linking may provide strength to the material. Fibers may have a range of diameters. In addition to type I collagen, skin (hides) may include other types of collagen as well, including type III collagen (reticulin), type IV collagen, and type VII collagen.
• Previous attempts to make engineered leathers have proven unsuccessful or impractical. For example,EP 1589098 ("the '098 application") describes a method of growing fibroblasts seeded onto three-dimensional bioactive scaffolds. The scaffolds may be made from collagen waste material from a tanning process ("split"), microparticles of pure collagen, particle of collagen waste material, or synthetic scaffolds (e.g., made of polymers such as HYAFF). The addition of the scaffold material complicates and increases the expense of their proposed process, and also affects the properties of any leather produced this way.
• The '098 application is one example of a scaffolding technique for culturing leather, however method such as this, which use a scaffold of waste or engineered collagen materials, have not been widely used because they are costly and difficult to work with, and have proven technically difficult to work with and commercialize.
• Described herein is a method of forming engineered leathers that may address the problems by forming materials akin to fiber reinforced composites (FRCs), in which cells are cultured on fibrous scaffolds (formed of fibers such as silk). In general, FRCs refer to composite building materials that form class of high performance materials used in a number of industries including energy, building, automotive, and sport. FRCs consist of a continuous matrix phase (typically a polymer matrix), a dispersed fiber phase (typically a stronger glass, carbon or cellulosic fiber), and an interface between the matrix and fibers. Within the interface, fibers adhere to the matrix phase through covalent or non-covalent interactions for effective force transmission from the matrix phase to the fibers. As a result, materials with incredible strengths can be realized that are not possible with each material individually.
• As mentioned, numerous tissue engineering scaffolds have been developed over the past 25 years to build biological tissues with defined structures and dimensions. These scaffolds provide surface area for cells to adhere and grow tissue in three dimensions. Fibrous materials consisting of entangled or woven fibers, 100nm-100um in diameter, have been widely explored as tissue engineering scaffolds due to their large surface areas for cell growth per unit volume, and high porosities to allow cell infiltration throughout the 3D scaffold architecture. Typically, tissue engineering scaffolds are biodegradable allowing the tissue to replace the scaffold as it grows. Therefore, the final product consists only of biological tissue to improve biocompatibility following implantation.
• Tissue engineering constructs are generally grown for biomedical applications, including insertion into a body to repair and/or replace biological tissue, thus biocompatibility has been an important consideration. However, the use of biological tissues for consumer goods applications requires a much different set of considerations. In such cases, the durability, appearance and ability to be tanned or preserved must be considered. Described herein are methods and techniques for the fabrication of biological tissue, that may address the concerns de-scribed above. In particular, described herein are methods of forming composites wherein the tissue is grown throughout a fibrous scaffold and cross-linked to the scaffold during a process analogous to tanning in order to create a novel class of high performance composites. These engineered leathers may replicate much of the structures and properties of natural leathers, but may be processed in a much simpler manner.
SUMMARY OF THE DISCLOSURE
• In general, described herein are methods of forming engineered, reinforced leather materials (engineered leathers) including a composite of a fibrous scaffold and a collagen network formed by cultured cells (e.g., fibroblasts). These composites are tanned to stabilize the collagen network and interactions between the fibrous scaffold and collagen network. These engineered leathers may be referred to as fiber-reinforced biological tissue composites.
• For example, a method of forming a fiber-reinforced biological tissue composite may be formed by culturing tissue-producing cells (e.g., fibroblasts) on a fibrous scaffold having a functional group selected from the group consisting of amine (-NH2), carboxylic acid (-COOH), sulfhydryl (-SH), and hydroxyl (-OH), and combinations thereof, which may be cross-linked (e.g., by tanning) to the tissue and/or proteins, such as collagen, formed and/or secreted by the cells and then tanning (e.g., chemically cross-linking). Tanning may be performed after the scaffold fibers have been at least partially covered in cultured cells and extracellular matrix released by the cultured cells.
• In general, because they are intended for use as part of a textile, any of the scaffolding materials described herein may be non-biodegradable, at least over the immediate term (e.g., within 1 year, 2 years, 5 years, 10 years, etc.) of ordinary use.
• As will be described in greater detail below, the density of tissue (e.g., cells and extracellular material deposited by the cells, particularly collagen) on the substrate should be greater than a minimum threshold in order for the resulting material (equivalently referred to herein as artificial leather, synthetic leather or cultured leather) that is greater than about 200,000 cells/cm² of substrate surface area immediately prior to tanning the material using any of the modified tanning (e.g., cross-linking) steps described herein. For example, the density of the cells may be between about 200,000 to about 4,500,000 cells/cm², or from about 250,000 to about 4,000,000 cells/cm², or from about 500,000 to about 2,000,000 cells/cm² of substrate surface area (e.g., between an upper and lower boundary where the lower boundary is 200,000 cells/cm², 250,000 cell/cm², 300,000 cells/cm², 400,000 cells/cm², 500,000 cell/cm², etc. of substrate surface area and the upper bound is 1,000,000 cells/cm², 1,500,000 cells/cm², 2,000,000 cells/cm², 3,000,000 cells/cm², 4,000,000 cells/cm², 5,000,000 cells/cm², 6,000,000 cells/cm², etc., of substrate surface area, where the upper bound is always greater than the lower bound). If the density is too low or too high, the leather will not have the desired quality/tensile strength. Once processed by cross-linking (tanning) and/or lubricating/fatliquoring, the material may comprise a tanned fibrous scaffolds comprising a plurality of fibers, wherein the fibers are surrounded by extracellular matrix cross-linked to the plurality of fibers at a density of greater than a minimum that is, e.g., 0.01 µm or greater (e.g., 0.02 µm or greater, 0.03 µm or greater, 0.04 µm or greater, 0.05 µm or greater, 0.06 µm or greater, 0.07µm or greater, 0.08 µm or greater, 0.09 µm or greater, 0.1 µm or greater, 0.2 µm or greater, 0.3 µm or greater, 0.4 µm or greater, 0.5 µm or greater, 0.6 µm or greater, 0.7 µm or greater, 0.8 µm or greater, 0.9 µm or greater, 1 µm or greater, 2 µm or greater, 3 µm or greater, 4 µm or greater, 5 µm or greater, 6 µm or greater,, 7 µm or greater, 8 µm or greater, 9 µm or greater, 10 µm or greater, etc., including between 0.01 µm and 200 µm, etc.), wherein there is good adhesion between the extracellular matrix and the plurality of fibers so that the tensile strength of the fiber-reinforced biological tissue composite material is greater than about 1.0 MPa. In general, these materials (artificial leathers) may be tanned an may have a water content that is less than 30%, less than about 25%, less than about 20%, less than about 18%, less than about 15%, less than about 12 %, less than about 10%, less than about 7%, less than about 5%, etc. Thus, the fiber-reinforced biological tissue composite material may be dehydrated.
• Although the amount of water in the initial culture is greater than 95%, this water must be removed to form the composite material (e.g., artificial leather, cultured leather, or synthetic leathers described herein). The tanning (cross-linking) process described herein typically include a dehydration/drying step that removes the majority of the water, so that the final amount of water left in the resulting composite material is between from about 1% to about 10% (e.g., from about 2% to about 8%, or from about 3% to about 7%, etc.) by weight. If the water content is too low, the resulting material (leather) will be brittle. If the water content is too high, the resulting material (leather) may be susceptible to microbial growth. In addition, the resulting material may be further processed to apply a lubricant material (e.g., by the addition and/or impregnation with a hydrophobic material (such as an oils, including sulfonated oils, wax, fat, etc.) such as may occur by a modified fatliquoring process. In any of the methods described herein a finishing step of impregnating the material (including soaking, coating, etc. with or without a pH and/or heating step) may be performed. They hydrophobic material (including one or more of oil, wax, fat, etc.) may be included in the final product in an amount that is between about 0.001% and about 15% (such as equal to or between about 0.1% and about 12%, e.g., having a lower bound of 0.001%, 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, etc. and an upper boundary of 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8%, etc., where the lower boundary is always less than the upper boundary) by weight. For example, any of the materials formed as described herein may include between about 0.1% and about 12% of a hydrophobic material (such as one or more of an oil, fat, wax, etc.).
• In general, the scaffolds used in the methods described herein are fibrous scaffolds formed of any material (including any cross-linkable material) that is conducive for cell growth and collagen tissue formation, but particularly protein materials (e.g., containing amine, carboxylic acid, sulfhydryl and hydroxyl groups), such as silk, in order to promote or stabilize interactions (both covalent and non-covalent) between the scaffold and the collagen tissue. Silk is an example of a fibrous scaffold. Silk is generally formed of a protein fiber that may be composed mainly of fibroin. For example, silk fibers from domesticated silkworms typically have a triangular cross section with rounded corners, 5-10 µm wide. The fibroin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations. Silkworm fibers are naturally extruded from two silkworm glands as a pair of primary filaments (brin), which are stuck together, with sericin proteins that act like glue, to form a bave. Bave diameters for tussah silk can reach 65 µm. Silk emitted by a silkworm may consist of two main proteins, sericin and fibroin, fibroin being the structural center of the silk, and serecin being the sticky material surrounding it. Fibroin is made up of the amino acids Gly-Ser-Gly-Ala-Gly-Ala and forms beta pleated sheets. Hydrogen bonds form between chains, and side chains form above and below the plane of the hydrogen bond network.
• Thus the scaffold may be a protein fiber containing amine and carboxylic acid groups, or a naturally occurring cellulose fibers containing (or modified to contain) amine and carboxylic acid groups. The fibers forming the scaffold may be chemically modified to enhance collagen (released by cultured cells) interactions with the scaffold. For example, the scaffold may be chemically modified to contain groups for crosslinking (covalent or non-covalently) with collagen including amines, carboxylic acids, sulfates, aldehydes, hydrazides, sulfhydryls, epoxides, acrylates, etc. These tissue crosslinking groups may be protected during tissue growth and activated for crosslinking when tissue growth is complete. In particular, the crosslinking referred to herein may be activated during tanning, which may be identical to, or derived from, traditional tanning methods and techniques, including omitting those steps which are made unnecessary by the use of tissue culture as described herein. Further, an additional reinforcement crosslinking step can be used to crosslink chemistries not involved in the traditional tanning process (anything other than amine and carboxylic acid groups). Traditional tanning chemicals can then be used to give the fiber reinforced biological tissue composite a leather-like aesthetic. Scaffolds may also be formed or (and/or may include) carbon fibers, which may also be modified as discussed above.
• In general, these tissue crosslinking groups may be pendant to the scaffold with a spacer between 10 daltons and 100 megadaltons. The scaffold may be crosslinked to the tissue through non-covalent interactions including ionic, hydrophobic and van der Waals forces. Alternatively or additionally, the scaffold may be cross-linked to the tissue through covalent bonds. For example, the scaffold may be directly reacted with amine, carboxylic acid and/or hydroxyl groups on the tissue. The scaffold may be reacted with a crosslinker which reacts with amine, carboxylic acid and/or hydroxyl groups on the tissue. The molecular weight of the crosslinker may be between 10 daltons and 100 megadaltons. The tissue referred to herein are the cultured cells and/or the products released by these cultured cells (e.g., extracellular matrix proteins, in particular collagen). Any of the crosslinkers described herein may include a functionality of the crosslinker of between 2 and 2000. In any of the materials formed as described herein a trace amount of the crosslinker used during the formation process may be found in the final material (e.g., less than 0.001%, less than 0.01%, less than 0.1%, etc.); alternatively no trace amount may be found. Thus, the material formed (the artificial leather) may have a residual amount of crosslinker present in the material, such as between 0.0001% and 1% (e.g., less than about 0.1%, less than 0.01%, less than 0.001%, etc., between 0.0001% and 0.01%, etc.).
• In general, the scaffold may be composed of fibers. Suitable fibers also include synthetic fibers. Suitable synthetic fibers include, but are not limited to polyester-polyurethane copolymers such as elastane or LYCRA^®, polyparaphenylene terephthalamide polymers such as KEVLAR^®, nylon polymers such as nylon 6, nylon 6,6 and the like, and polyester polymers such as polyethylene terephthalate. The fibers may be an appropriate size or dimension (e.g., the fibers may have a length of between about 100 nm to 1 m). The fibers may be assembled in a woven or non-woven architecture (or a combination of both). The density of fibers in the scaffold may be between 10 and 10,000 mg/cc. The porosity of the fibrous scaffold may be between 10 and 99%.
• Any appropriate cells may be cultured on the fibrous scaffold. The cells may originate from a tissue and/or cell-line. For example, the cells may be of mammalian origin (e.g., bovine, porcine, ovine, etc.). The cells may be of reptile origin (e.g., snake, lizard, etc.). The cells may be of bird origin (e.g., chicken, ostrich, turkey, etc.). The cells may be of fish origin (e.g. shark, etc.). The cells may be of amphibian origin (e.g. frog, salamander, etc.). The cells may be genetically modified (e.g., to increase production of extracellular matrix "ECM", including, e.g., collagen, etc.) or they may be unmodified.
BRIEF DESCRIPTION OF THE DRAWINGS
• The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which:
• FIG. 1A show a first view of an example of a fibrous scaffold formed of silk fibers that may be used in a method to form the composite manufactured leather described herein. InFIG. 1A the image is shown at a low magnification (scale bar is 1 mm).
• FIG. 1B shows the fibrous scaffold ofFIG. 1A at higher magnification (scale bar is 0.5 mm).
• FIGS. 2A and 2B illustrate tissue growth on a fibrous scaffold such as the silk scaffold shown inFIGS. 1A-1B. FIG. 2A shows the fibrous scaffold andFIG. 2B shows the fibrous scaffold ofFIG. 2A following four weeks of fibroblast culture. Cells were seeded onto the silk fiber scaffold shown inFIG. 2A and after four weeks of culture the scaffold fibers are surrounded by tissue (FIG. 2B).
• FIG. 3 shows an example of a silk scaffold onto which fibroblasts have been cultured, prior to tanning. InFIG. 3, a section of scaffold onto which fibroblasts have been cultured (e.g., for four weeks) has been stained with picrosirus red to visualize collagen.
• FIG. 4 is a scanning electron micrograph showing a portion of a fibrous silk scaffold onto which fibroblasts have been grown and allowed (and in some variations stimulated) to secrete collagen. Collagen rich tissue has grown throughout the silk fiber scaffold.
• FIG. 5 shows examples of four different composites of fibrous scaffolds (silk, high density PLLA, polyester, low density PLLA) and fibroblasts after eight weeks of culture, followed by tanning. Only the fibrous silk scaffold composite was successfully tanned into a leather like material. This could be due to the presence of amine, carboxylic acid and hydroxyl groups on the silk protein, although differences in the amount and type of tissue formed on the different scaffolds cannot be ruled out.
• FIG. 6 shows an example of the leather-like surface of a fibrous silk scaffold composite after tanning.
• FIG. 7 is an example of the edge of a fibrous silk scaffold composite after tanning, in which a gradient of tissue was produced towards the edge of the silk scaffold, revealing the silk fibers dispersed throughout the tissue matrix.
DETAILED DESCRIPTION
• Described herein are methods of forming engineered leathers using a fibrous scaffold that is tanned (e.g., cross-linked) to the cultured cells and/or any extracellular matrix (ECM) components released by the cultured cells to form. The resulting engineered leather may be referred to as a fiber-reinforced tissue composite and may have superior properties (e.g., durability, strength, etc.) compared to other engineered leathers. Also described are methods of growing/culturing fiber-reinforced tissue composites and methods of tanning them.
• In general, the scaffolds described herein are configured to be cross-linked to the released extracellular matrix (e.g., collagen, fibronectin, fibrin, hyaluronic acid, proteoglycans, etc.) from cells cultured on the scaffold. These scaffolds may be formed of a material capable of forming cross-links with ECM and/or the cells during tanning. Further, the structure of the scaffold (e.g., porosity, fiber length, fiber density, etc.) may be chosen to permit cross-linking and/or to encourage growth of the cells and release of extracellular matrix. The scaffold is tanned with and to the ECM to form the final product, the fiber-reinforced tissue composite. Thus, the scaffold forms an integral part of the final product, and its dimensions, including the thickness, may help determine the final thickness of the resulting leather.
• One particular example of a fibrous scaffold that may be used as described herein is silk. Silk (e.g., organic and/or synthetic silk) may be formed (e.g., spun) to a predetermine fiber thickness and used in a woven and/or non-woven sheet forming the scaffold onto which ECM-releasing cells may be cultured. Such cells may be, e.g., dermal fibroblasts, smooth muscle cells, etc.
• In general, the scaffold is formed of a material (such as silk) that is conducive to cell attachment and tissue growth. In addition the scaffold material can contain surface chemistries that adhere the scaffold fiber to the tissue for effective stress transfer in the final material. Interactions between the scaffold fibers and the tissue can be non-covalent, including electrostatic, hydrophobic, dipole-dipole, induced dipole or van der Walls interactions. These interactions can be tuned through scaffold fiber surface chemistry. In addition, covalent interactions or crosslinks can be introduced to form a chemical bond between scaffold fiber and tissue.
• The fibrous scaffold may provide a great deal of surface area onto which cells may be cultured. For example, the scaffold may be formed of relatively dispersed fibers forming a dispersed fibrous scaffold. A fibrous scaffold may have a lot of surface area to allow cell growth and to allow for a strong fiber-tissue matrix interface (which may also enhance overall strength of the resulting composite). In some variations, the length of the fibers forming the scaffold is between 100 nm and 100 µm. In some variations, the length of the fibers is between 100 nm and up to 1 mm, 10 mm, 100 mm or 1 m. The density of the fibrous material in the scaffold may be between 10 and 100 mg/cc. In some variations the density of the fibrous material is between 10 and 10,000 mg/cc. The porosity of the fibrous scaffold (including of the woven/non-woven fibers) may be between 10 and 99%.
• In general, it may be advantageous to provide a high porosity. This may permit a lot of cell infiltration and tissue growth, including within the thickness of the scaffold. The scaffold may be any appropriate thickness (e.g., between 10 µm and 5 mm, e.g., between 100 µm and 1 mm, between 100 µm and 500 µm, between 50 µm and 300 µm, etc., or between any lower value of 10 µm, 30 µm, 50 µm, 75 um, 100 um, 200 um, 300 µm, etc. and an upper value of 50 µm, 100 µm, 150 µm, 200 µm, 300 µm, 400 µm, 500 µm, 600 µm, 700 µm, 800 µm, 900 µm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, etc.). The porosity may be determined as the space (e.g., average diameter) between fibers.
• In operation, the method of forming the fiber-reinforced tissue composites described herein may include culturing cells on the fibrous scaffold until a desired amount of ECM (e.g., collagen) has been formed on and/or over the fibrous scaffold. Optionally, the fibrous scaffold may be formed and/or prepared to receive the cells to be cultured thereon. For example, the scaffold may be treated to expose reactive groups or cell-attachment sites. In some variations the fibrous scaffold may be applied in a cell reactor for seeding and growth of the cells on the scaffold. As an example, cells, which may be, for example, a mammalian cell line having known ECM releasing properties, be seeded onto the prepared fibrous scaffold and allowed to grow, divide and set down ECM. Cells may be cultured in typical culture conditions for an appropriate length of time (e.g., 2-90 days). Additional cells may be added during this period. Thereafter, the resulting material, comprised of the fibrous scaffold with released ECM and cells, may be referred to as an intermediate (untanned) tissue construct, which may be further processed. This intermediate (untanned) tissue construct may be decelluarlized (e.g., by treatment with alcohols or other agents) or not. The intermediate (untanned) tissue construct may then be processed by tanning, which may include traditional tanning chemistries as well as crosslinking reactions to reinforce the scaffold to the tissue.
• Provided below is an example of various reinforcement and tanning methods that may be used and/or modified. The fibrous scaffold and the cultured ECM grown on the scaffold are tanned (and adhered) together by this process, and the resulting fiber-reinforced tissue composites may be used as an engineered leather material.
• A reinforcement step may be included to crosslink the deposited ECM to the fibrous scaffold encased in ECM, followed by a tanning procedure to produce the look and feel of traditional leather. While traditional tanning chemistries may effectively crosslink ECM to scaffold if the scaffold has proper functionality (such as surface amine and carboxylic acid groups), other scaffold surface chemistries and cross linkers may be incorporated to strengthen the ECM-scaffold interface. These include epoxide, acrylate, aldehyde, sulfhydryl, diazirines, aryl-azides, etc., as well as protected chemistries that may be activated following tissue growth to reduce cytotoxicity of the scaffold surface. The scaffold may be modified with reactive chemistries prior to cell seeding that are pendant from spacer molecules of size between 0 Da and 100MDa. The scaffold surface chemistry may be reacted directly to the ECM or through cross linkers with functionality between 1 and 2000 and with size between 1Da and 100MDa. Further, crosslinking molecules may be polymerized within the scaffold-ECM construct with a size between 1Da and 100MDa. Polymerization of these cross linkers may be initiated following tissue growth with triggers such as light exposure, temperature change, addition of chemical initiators, etc.
• In general, any appropriate tanning methods may be used, including method derived from traditional tanning, which may result in leather having the look and feel of traditional tanned hides. For example, following culturing of cells on the scaffolds as described above (e.g., in a sterile environment), the intermediate (untanned) tissue construct consisting of cells and ECM grown on the planar (e.g., relatively thin, but long and wide sheet of) fibrous scaffold may be washed (e.g., to remove culture medium), and tanned. Traditional steps such as liming (e.g., treatment with a basic compound such as milk of lime and/or the addition of "sharpening agents" including disulfide reducing agents such as sodium sulfide, cyanides, amines, etc.) may be included. Such steps may be modified, however, as the intermediate construct does not include hair, nail and other keratinous matter typically found in native skins. Such steps may be maintained or modified to remove some ECM materials and/or to swell and split fibers in the intermediate construct or otherwise prepare collagen in the construct for tanning.
• The methods may include or avoid (as unnecessary) the use of unhairing agents such as sodium sulfide, sodium hydroxide, sodium hydrosulfite, calcium hydrosulfide, dimethyl amine, and sodium sulfhydrate.
• A step that removes cellular material while preserving the ECM may also be included. Decellularization methods used in tissue engineering may be used for this purpose including the use of surfactants, enzymes, ultrasonic energy, freeze thaw cycles, etc.
• A deliming step may also be included. For example, the pH of the collagen may be brought down to a lower level so that enzymes may act on it. Depending on the end use of the leather, the intermediate construct may be treated with enzymes to soften them, a process called bating. If bating is used, once completed, the intermediate tissue construct may be treated first with salt and then with sulfuric acid, if a mineral tanning is to be done, which may bring down the pH of collagen to a very low level so as to facilitate the penetration of mineral tanning agent into the substance. This process is known as pickling. The salt (e.g., sodium chloride) may penetrate faster than the acid and limit the ill effect of sudden drop of pH. If vegetal tanning is used, the tanning agent may be tannin. The tannins are a class of polyphenol astringent chemicals that occur naturally in the bark and leaves of many plants. Tannins bind to the collagen proteins and may hide and coat them, causing them to become less water-soluble and more resistant to bacterial attack. The process may also cause the material to become more flexible. Traditionally, primary barks, processed in bark mills and used in modern times, are chestnut, oak, redoul, tanoak, hemlock, quebracho, mangrove, wattle, and myrobalans. Traditionally, hides are stretched on frames and immersed for several weeks in vats of increasing concentrations of tannin. The intermediate tissue constructs described herein may provide easier, more direct access to the tannins and may therefore require less processing time in general.
• In chrome tanning, prior to the introduction of the basic chromium species, several steps may be used to prepare the material, as mentioned above, including the introduction of alkali agents such as sodium hydroxide, restoring neutral pH, bating (softening with enzymes), and pickling (lowering pH of the material being processed, e.g., with salt and sulfuric acid).
• In traditional tanning, the pH is very acidic when the chromium is introduced, to ensure that the chromium complexes are small enough to fit in between the fibers and residues of the collagen. Once the desired level of penetration of chrome into the substance is achieved, the pH of the material is raised again to facilitate the process. This step is known as basification. Chrome tanning is typically faster than vegetable tanning.
• Chromium(III) sulfate ([Cr(H₂O)₆]₂(SO₄)₃) has long been regarded as the most efficient and effective tanning agent. Chromium(III) compounds of the sort used in tanning are significantly less toxic than hexavalent chromium. Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation, [Cr(H₂O)₆]³⁺, which at higher pH undergoes processes called olation to give polychromium(III) compounds that are active in tanning, being the cross-linking of the collagen subunits. The chemistry of [Cr(H₂O)₆]³⁺ is complex due to the presence of a variety of ligands. Some ligands include the sulfate anion, the collagen's carboxyl groups, amine groups from the side chains of the amino acids, and masking agents. Masking agents are carboxylic acids, such as acetic acid, used to suppress formation of polychromium(III) chains. Masking agents allow the tanner to further increase the pH to increase collagen's reactivity without inhibiting the penetration of the chromium(III) complexes.
• As mentioned above, collagen is characterized by a high content of glycine, proline, and hydroxyproline, usually in the repeat -gly-pro-hypro-gly-. These residues give rise to collagen's helical structure. Collagen's high content of hydroxyproline allows for significant cross-linking by hydrogen bonding within the helical structure. Ionized carboxyl groups (RCO2-) are formed by hydrolysis of the collagen by the action of hydroxide. This conversion occurs during the liming process, before introduction of the tanning agent (chromium salts). The ionized carboxyl groups coordinate as ligands to the chromium(III) centers of the oxo-hydroxide clusters.
• Tanning increases the spacing between protein chains in collagen from 10 to 17 Å. The difference is consistent with cross-linking by polychromium species, of the sort arising from olation and oxolation.
• Subsequent to application of the chromium agent, the bath may be treated with sodium bicarbonate to increase the pH to 4.0-4.3. This increase induces cross-linking between the chromium and the collagen. The pH increase may normally be accompanied by a gradual temperature increase up to 40°C. Chromium-tanned leather can contain between 4 and 5% of chromium. This efficiency is characterized by its increased hydrothermal stability of the leather, and its resistance to shrinkage in heated water.
• Other forms of tanning may be used, including ones based on alum, zirconium, titanium, iron salts, or a combination thereof. Tawing may be used on the intermediate tissue constructs described herein. Tawing is a method that uses alum and aluminum salts, generally in conjunction with other products such as egg yolk, flour, and other salts. The material becomes tawed by soaking in a warm potash alum and salts solution (or equivalent), between 20 and 30°C. The process may increase the resulting leather's pliability, stretchability, softness, and quality. Adding egg yolk and flour (or equivalents) to the standard soaking solution may further enhance its fine handling characteristics. Then, the intermediate tissue construct is dried and allowed to stabilize.
• Depending on the finish desired, the material may be waxed, rolled, lubricated, oiled (e.g., injected with oil), and/or dyed. Suedes, nubucks, etc. may be formed, e.g., by inducing surface finishes. The material may be additionally finished by retawing. Retawing agents and/or dyes may be applied to the material to enhance the physical strength and properties desired depending on the end product. A final stage, finishing, may be used to apply finishing material to the surface or finish the surface.
• Once cells on the fibrous scaffold have been grown to the proper density so that the resulting material (once tanned) to have the desired strength (e.g., greater than about 200,000 cells/cm² of substrate surface area) and the 'wet' material has been formed, it may be tanned, as described above, using any of the tanning (or modified tanning procedures mentioned. In addition, the material is typically treated by one or more other post-culturing processes (which may be performed with the tanning steps mentioned above, or after the tanning steps).
• For example, fatliquoring may include the addition of natural or synthetic lubricants to the fibers prior to forming them into a textile, which not only allow the fibers of the material to dry without interfacial adhesion (sticking) but may also provide hydrophobicity, and other properties, to the material. The methods described herein may provide advantages over regular fatliquoring, in that the material, which may have a predictable and highly regular structure/fiber density, may be definitively treated with a set amount of lubricant (hydrophobic material, e.g., oil, wax, fat, etc.) to ensure uniform and/or regular penetration, which is not guaranteed with natural leather due to the variability in internal fiber structure. These methods may also allow for alternate lubricants to be used that could not be normally considered due to issues with dispersion size, i.e., in natural leather for deep fiber penetration the emulsion size may be critical in order to be small enough to penetrate fully within the fiber matrix, but this may be alleviated by treating fibers individually as described herein. Further these methods may also provide improved tensile and tearing strength characteristics due to highly efficient fatliquoring. Finally the methods described herein may provide nearly complete (e.g., 100%) efficiency in exhaustion of the reagent, including the lubricant; in addition, the lubricant may be virtually immediately reacted with the material, as opposed to having to penetrate and then fix the lubricant as required by in natural leather. This results in an extremely energy efficient and material efficient process, using less chemicals, lower temperatures (as 'cold' fatliquoring may be employed or alternatively hot/warming of the material may be performed) and reduced water usage which consequently will reduce effluent requirements. In some variations, the percentage of fatliquoring (e.g., the percentage of oil lubricant by weight) may be less than 12% (e.g., between 0.1-15%, between 0.5-10%, etc. by weight of the material). In addition, one or more lubricants may be incorporated into the material itself during the formation process, including prior to tanning. In some variations the fatliquoring may be performed immediately after tanning the material. Examples of fatliquoring agents may include: oils (e.g., sulfonated oils, mineral oil, etc.), fats (animal fats, vegetable fats, e.g., glycerides, etc.), synthetic lubricants, polysiloxanes, lubricating acrylic polymers, dry lubricants, etc. The fibers and/or a textile made from the fibers described herein may be made water resistant (e.g., "waterproofing") by the addition of an agent such as a hydrophobic agent, which can include, but are not limited to, hydrophobic lubricants (e.g. a modified polysiloxane such as Densodrin CD from BASF), fluorocarbons, hydrophobic acrylic polymers, chromium stearates, etc. The ability to make each fiber water resistant may also increase the consistency and varying levels of water resistance may be achievable in a controlled manner.
• Interestingly, in some variations, the materials formed according to the methods described herein may be used to form a cutured leather without fatliquoring as a separate, post-culturing step. For example the fibrous tissue scaffold material may include a material that provides sufficient moisture retention and/or hydrophobicity to avoid the necessity for additional fatliquoring. For example, the fibers forming the scaffolding material may be hydrophobic. Mildly hydrophobic tissue scaffolds may include polymers (e.g., aliphatic polyesters, etc.).
• Retanning is performed with traditional leather materials to modify the qualities of the leather, including increasing/decreasing the concentration of the tanning agent, and/or modify the properties of the resulting fibers and/or textiles which may in term enhance further processing of the material, including dying. In any of the methods described herein retanning may be performed with the same or a different tanning agent. Retanning may be performed on the material. Retanning may be combined with any of the other steps described herein, including fatliquoring and/or dyeing. In addition, the order of these steps may be performed in any appropriate sequence (e.g., retanning then fatliquoring, etc.).
• Dyeing adds color to the fibers and/or the resulting textile. Any appropriate dye may be used, particularly dyes that are appropriate for leather (e.g., collagen materials although other dyes specifically designed for textiles are may be used including those that have compatible reactivity, e.g. reactive dyes. As mentioned, in some variations a dye or dyes may be included in the culturing solution prior to tanning or immediately after tanning. Dyes may include acid dyes (e.g., pre-metallized acid dyes), basic dyes, direct dyes, reactive dyes and sulfur dyes. A mordant dye (e.g., including a mordant to help with binding of the dye to the material) may also be used.
• Other chemical treatments may also be added such as: flame retardants, abrasion resistance treatments, thermos-regulating technologies, moisture management technologies, performance particulates, etc.
Examples
• FIGS. 1-6 illustrate methods of forming fiber-reinforced tissue composites as described above.
• For example,FIGS. 1A and 1B show an example of a fibrous tissue scaffold that may be used. In this example, the scaffold is formed of silk fibers. The scaffold has a thickness of about 0.5 mm, and fiber diameters of about 15-20 microns (e.g., on average/mean). In this example, the overall material density is approximately 80 mg/cc, which corresponds to a porosity of about 95%. The orientation of fibers may affect overall materials properties, and the assembly process may be varied to produce a variety of non-woven, woven and knit architectures. In addition, the length of the fibers may affect overall material properties, and the fiber length, diameter and/or porosity may be varied. For example, microfibers may be used having a diameter from about 10 µm. (microns) to 20 µm or more.FIG. 1B shows an enlarged view of a region of the scaffold shown inFIG. 1A.
• FIG. 2A shows an example of a "dry" fibrous (silk) scaffold prior to seeding with cells. InFIG. 2B, the scaffold shown inFIG. 2A has been seeded with cells (in this example, bovine dermal fibroblasts) at density of 8.5x10⁶ cells/ml. The volume of the seeded cells was 100 µL/cm² of scaffold (similar to that shown inFIG. 1A-1B). In this example, cells are seeded on top of the scaffold and have been seen to settle (e.g., via gravity) onto the porous scaffold. Thereafter, the scaffold can be turned, flipped, rotated, etc., which may help distribute the cells and/or culture medium throughout the substrate. In practice it may be beneficial to have a fibrous substrate that is initially greater than 50% porous. As mentioned above, in some variations, the scaffold may be modified to enhance cell attachment.
• In the example, shown inFIG. 2B, the cells may be cultured for approximately 4 weeks, with regular change in media (e.g., every few days); standard tissue culture techniques may be adapted for use with the scaffold; for example, growth factors may be used, or agents to enhance the release and/or type of ECM deposited. In addition, dynamic culture environments such as perfusion or mechanical loading may be used to enhance ECM deposition. InFIG. 2A-2B, the scaffold has a thickness of about 0.5 mm. Thicker scaffolding may also be used (e.g., up 5 cm). After four weeks of culture the scaffold fibers are surrounded by tissue (including collagen).
• FIG. 3 shows a section taken from the exemplary intermediate (untanned) tissue construct shown inFIG. 2B in cross-section through the tissue. In this example the stain shows (red in original color version) collagen; scaffold fibers are indicated (in section) by the arrowheads. The sample is shown having a dense network of collagen extending around and through (e.g., within the porous fibrous scaffold). This sample was fixed for histology. A scanning electron microscopic image of an intermediate (untanned) tissue construct similar to that shown inFIG. 2B is provided inFIG. 4.
• InFIG. 4, a section through a fibrous scaffold onto which cells releasing collagen have been cultured is shown. The fibrous scaffold in this example is silk, and the SEM shows collagen-rich tissue grown throughout the silk fiber scaffold.
• FIG. 5 shows a comparison between different fibrous scaffolds for tissue culture and subsequent tanning into a leather-like material. InFIG. 5 the silk scaffold was much more successful than the polylactic acid, PLLA, scaffold; the silk scaffold allowed reinforcement crosslinking during a tanning process as described herein. InFIG. 5, cells have been cultured equivalently on four different fibrous scaffolds and tanned:silk 501,high density PLLA 503,polyester 505, andlow density PLLA 507. Only the silk scaffold included amine and carboxylic acid groups which may crosslink with the tissue/ECM during the tanning process (chromium salt tanning). There are also differences in the fiber density and ability of these materials to promote tissue growth. These scaffolds were each about 4 cm long and about 1.5 cm wide; thesilk 501 andlow density PLLA 507 are each 0.5 mm thick and the high density PLLA and polyester are each about 1 mm thick. Following tanning, only samples in which the scaffold was cross-linkable when tanning (e.g., silk 501) resulted in materials having a texture and look similar to native leather. Tanned silk without cells cultured on it does not look similar to native leather; the resulting material (silk alone) pulls apart.
• FIGS. 6 and 7 illustrate example of fiber-reinforced tissue composites formed as described herein after tanning.FIG. 6 shows an outer surface of a tanned material that has been formed and processed as described above. In this example, the leather-like outer surface feels similar to native leather. The material is formed of a silk fibrous scaffold onto which cells (dermal fibroblasts) have been cultured, and the resulting intermediate tissue construct has been tanned.FIG. 7 shows an enlarged view of an edge region of a tanned (silk) fiber-reinforced tissue construct. After tanning, a gradient of tissue was produced toward the edge of the silk scaffold, revealing the silk fibers dispersed through the tissue matrix (on the right in the figure).
• In general, the resulting product will have a look and feel similar to native leather, but is detectably (e.g., under examination of the ultrastructure) different. For example, the resulting material will have the fibrous scaffold material dispersed throughout, and typically surrounded by ECM (e.g., collagen); the collagen and the scaffold are both tanned and adhered together. There will typically be an outer layer of tissue on the scaffold. The sample may be tested for tensile strength using and Instron machine. Clamps are attached to the ends of the sample and the sample is pulled in opposite directions until failure. Good adhesion is demonstrated when the sample has a tensile strength of at least 1 MPa. In any of the variations described herein, the final material (leather) produced as described herein may have a desirable range of properties due to the unique method of processing/manufacture described herein. For example, in any of the materials described herein the material may have a tensile strength (estimated as described above) of greater than about 0.5 MPa, greater than about 0.6 MPa, greater than about 0.7 MPa, greater than about 0.8 MPa, greater than about 0.9 MPa, greater than about 0.95 MPa, greater than about 1.0 MPa, greater than about 1.1 MPa, greater than about 1.2 MPa, greater than about 1.3 MPa, greater than about.4 MPa, greater than about 1.5 MPa, etc.
• When a feature or element is herein referred to as being "on" another feature or element, it can be directly on the other feature or element or intervening features and/or elements may also be present. In contrast, when a feature or element is referred to as being "directly on" another feature or element, there are no intervening features or elements present. It will also be understood that, when a feature or element is referred to as being "connected", "attached" or "coupled" to another feature or element, it can be directly connected, attached or coupled to the other feature or element or intervening features or elements may be present. In contrast, when a feature or element is referred to as being "directly connected", "directly attached" or "directly coupled" to another feature or element, there are no intervening features or elements present. Although described or shown with respect to one embodiment, the features and elements so described or shown can apply to other embodiments. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed "adjacent" another feature may have portions that overlap or underlie the adjacent feature.
• Terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. For example, as used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items and may be abbreviated as "/".
• Spatially relative terms, such as "under", "below", "lower", "over", "upper" and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device in the figures is inverted, elements described as "under" or "beneath" other elements or features would then be oriented "over" the other elements or features. Thus, the exemplary term "under" can encompass both an orientation of over and under. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Similarly, the terms "upwardly", "downwardly", "vertical", "horizontal" and the like are used herein for the purpose of explanation only unless specifically indicated otherwise.
• Although the terms "first" and "second" may be used herein to describe various features/elements (including steps), these features/elements should not be limited by these terms, unless the context indicates otherwise. These terms may be used to distinguish one feature/element from another feature/element. Thus, a first feature/element discussed below could be termed a second feature/element, and similarly, a second feature/element discussed below could be termed a first feature/element without departing from the teachings of the present invention.
• Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising" means various components can be co-jointly employed in the methods and articles (e.g., compositions and apparatuses including device and methods). For example, the term "comprising" will be understood to imply the inclusion of any stated elements or steps but not the exclusion of any other elements or steps.
• As used herein in the specification and claims, including as used in the examples and unless otherwise expressly specified, all numbers may be read as if prefaced by the word "about" or "approximately," even if the term does not expressly appear. The phrase "about" or "approximately" may be used when describing magnitude and/or position to indicate that the value and/or position described is within a reasonable expected range of values and/or positions. For example, a numeric value may have a value that is +/- 0.1% of the stated value (or range of values), +/- 1% of the stated value (or range of values), +/- 2% of the stated value (or range of values), +/- 5% of the stated value (or range of values), +/-10% of the stated value (or range of values), etc. Any numerical values given herein should also be understood to include about or approximately that value, unless the context indicates otherwise. For example, if the value "10" is disclosed, then "about 10" is also disclosed. Any numerical range recited herein is intended to include all sub-ranges subsumed therein. It is also understood that when a value is disclosed that "less than or equal to" the value, "greater than or equal to the value" and possible ranges between values are also disclosed, as appropriately understood by the skilled artisan. For example, if the value "X" is disclosed the "less than or equal to X" as well as "greater than or equal to X" (e.g., where X is a numerical value) is also disclosed. It is also understood that the throughout the application, data is provided in a number of different formats, and that this data, represents endpoints and starting points, and ranges for any combination of the data points. For example, if a particular data point "10" and a particular data point "15" are disclosed, it is understood that greater than, greater than or equal to, less than, less than or equal to, and equal to 10 and 15 are considered disclosed as well as between 10 and 15. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed.

Claims (8)

1. A method of forming a fiber-reinforced biological tissue composite, the method comprising:

   culturing cells on a fibrous scaffold until fibers of the fibrous scaffold are surrounded by tissue comprising extracellular matrix at a density of greater than about 200,000 cells/cm² of substrate surface area, forming a tissue construct; and

   tanning the tissue construct to form a fiber reinforced leather-like material.
2. The method of claim 1, wherein culturing comprises culturing cells on

   - a fibrous silk scaffold,

   - a fibrous protein scaffold, wherein the fibrous protein scaffold comprises one or more of: a fibrous wool scaffold, a fibrous silk scaffold, a fibrous casein scaffold, a fibrous fibrin scaffold, or a fibrous collagen scaffold

   - a protein fiber scaffold comprising amine, carboxylic acid, hydroxyl and sulfhydryl groups

   - a fibrous cellulosic scaffold, wherein the fibrous cellulosic scaffold comprises one or more of a fibrous flax scaffold, a fibrous pineapple scaffold, a fibrous tunicate scaffold or a fibrous cotton scaffold, and/or

   - a synthetic fiber scaffold that facilitates cell attachment and tissue growth, wherein the fibrous synthetic scaffold comprises one or more of a carbon fiber scaffold, a Kevlar scaffold, a lycra scaffold, a nylon scaffold, a polyester scaffold, or a fiberglass scaffold.
3. The method of any one of the foregoing claims, wherein culturing comprises culturing the cells on the fibrous scaffold until the fibers are surrounded by collagen.
4. The method of any one of the foregoing claims, wherein culturing comprises culturing cells on the fibrous scaffold

   - comprising fibers having a length of between about 100 nm and 1 m, and/or

   - having a porosity of between 10 and 99%.
5. The method of any one of the foregoing claims, wherein culturing comprises culturing mammalian cells on the fibrous scaffold.
6. The method of any one of the foregoing claims, wherein tanning comprises crosslinking amine, hydroxyl, sulfhydryl, tyrosyl, and carboxylic acid groups on the extracellular matrix proteins in the tissue to the scaffold.
7. The method of any one of the foregoing claims, wherein tanning comprises crosslinking the fibers of the fibrous scaffold to the tissue through covalent bonds.
8. The method of any one of the foregoing claims, wherein tanning comprises crosslinking the fibers of the fibrous scaffold to the tissue through ionic, hydrophobic and van der Waals interactions.

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